t/eml.t: workaround newer Email::MIME* behavior
Recent (2020) versions of Email::MIME (and/or dependencies)
have different behavior than historical versions which seem
to be less DWIM and perhaps technically more correct. We'll
retain historical behavior for now, since it doesn't seem to
cause real problems and DWIM-ness is often required to make
sense of historical mail.
Tested on a FreeBSD 11.4 VM with the following packages:
p5-Email-MIME-1.949
p5-Email-MIME-ContentType-1.024_1
p5-Email-MIME-Encodings-1.315_2
(cherry picked from commit
5c4874309c4dd7aa8b9acd6bb97bd512360ab08a)